fix sphinx version

requirements/base.txt

@@ -41,7 +41,7 @@ versioneer==0.28
 
 # Online documentation via django-docs
 docutils==0.18.1
-Sphinx==7.1.2
+Sphinx==6.2.1  # NOTE: sphinx-rtd-theme v1.2.2 forces <7
 django-docs==0.3.3
 sphinx-rtd-theme==1.2.2
 sphinxcontrib-youtube==1.2.0